1. Field of Endeavor
The invention relates to a premix burner having a mixing section for a heat generator, preferably for a combustion chamber for operating a gas turbine plant, having sectional conical shells which complement one another to form a swirl body, enclose a conically widening swirl space and mutually define tangential air-inlet slots, along which feeds for gaseous fuel are provided in a distributed manner, having at least one fuel feed for liquid fuel, this fuel feed being arranged along a burner axis passing centrally through the swirl space, and having a mixing tube adjoining the swirl body downstream via a transition piece.
2. Brief Description of the Related Art
Premix burners of the generic type have been successfully used for many years for the firing of combustion chambers for driving gas turbine plants and constitute largely perfected components with regard to their burner characteristics. Depending on use and desired burner outputs, premix burners of the generic type are available which are optimized both with regard to burner output and from the aspect of reduced pollutant emission.
A premix burner without a mixing tube, which premix burner is to be briefly referred to on account of the development history, can be gathered from EP 0 321 809 B1 and essentially includes two hollow, conical sectional bodies which are nested one inside the other in the direction of flow and whose respective longitudinal symmetry axes run offset from one another, so that the adjacent walls of the sectional bodies form tangential slots in their longitudinal extent for a combustion air flow. Liquid fuel is normally sprayed via a central nozzle into the swirl space enclosed by the sectional bodies, whereas gaseous fuel is introduced via the further nozzles present in longitudinal extent in the region of the tangential air-inlet slots.
The burner concept of the foregoing premix burner is based on the generation of a closed swirl flow inside the conically widening swirl space. However, on account of the increasing swirl in the direction of flow inside the swirl space, the swirl flow becomes unstable and turns into an annular swirl flow having a backflow zone in the flow core. The location at which the swirl flow, due to breakdown, turns into an annular swirl flow having a backflow zone, with a “backflow bubble” being formed, is essentially determined by the cone angle which is inscribed by the sectional conical shells, and by the slot width of the air-inlet slots. In principle, during the selection for dimensioning, the slot width and the cone angle, which ultimately determines the overall length of the burner, narrow limits are imposed, so that a desired flow zone can arise which leads to the formation of a swirl flow which breaks down in the burner orifice region into an annular swirl flow while forming a spatially stable backflow zone in which the fuel/air mixture ignites while forming a spatially stable flame. A reduction in the size of the air-inlet slots leads to an upstream displacement of the backflow zone, as a result of which, however, the mixture of fuel and air is ignited sooner and further upstream.
On the other hand, in order to position the backflow zone further downstream, i.e., in order to obtain a longer premix or evaporation section, a mixing section, transmitting the swirl flow, in the form of a mixing tube is provided downstream of the swirl body as described in detail, for example, in EP 0 704 657 B1. Disclosed in that publication is a swirl body which consists of four conical sectional bodies and adjoining which downstream is a mixing section serving for further intermixing of the fuel/air mixture. For the continuous transfer of the swirl flow, discharging from the swirl body, into the mixing section, transition passages running in the direction of flow are provided between the swirl body and the mixing section, these transition passages serving to transfer the swirl flow formed in the swirl body into the mixing section arranged downstream of the transition passages.
However, the provision of a mixing tube inevitably reduces the size of the backflow bubble, especially since the swirl of the flow is to be selected in such a way that the flow does not break down inside the mixing tube. The swirl is consequently too small at the end of the mixing tube for a large backflow bubble to be able to form. Even tests for enlarging the backflow bubble in which the inner contour of the mixing tube provides a diffuser angle opening in a divergent manner in the direction of flow showed that such measures lead to the upstream drifting of the flame. Furthermore, additional problems arise with regard to flow separations close to the wall along the mixing tube, these flow separations having an adverse effect on the intermixing of the fuel/air mixture.
In addition to the mechanical design of the burner, the feeding of fuel also has a decisive effect on the flow dynamics of the swirl flow forming inside the swirl body and of the backflow bubble forming as far as possible in a stable manner in the space downstream of the swirl body. Thus, a rich fuel/air mixture forming along the burner axis is found during typical feeding of liquid fuel along the burner axis at the location of the cone tip of the conically widening swirl space, in particular in premix burners of a larger type of construction, as a result of which the risk of “flashback” into the region of the swirl flow increases. Such flashbacks firstly lead inevitably to increased NOX emissions, especially since the fully intermixed portions of the fuel/air mixture are burned as a result. Secondly, flashback phenomena in particular are dangerous and are therefore to be avoided since they may lead to thermal and mechanical loads and consequently to irreversible damage to the structure of the premix burner.
A further very important, environmental aspect relates to the emission behavior of such premix burners. It is known from various publications, for example from Combust. Sci. and Tech. 1992, Vol. 87, pp. 329-362, that, although the size of the backflow bubble in the case of a perfectly premixed flame has no effect on the NOX emissions, it is able to considerably influence the CO, UHC emissions and the extinction limit; i.e., the larger the backflow zone, the lower the CO, UHC emissions and the extinction limit. With a flame stabilization zone or backflow bubble forming to a greater extent, a larger load range in the premix burner can therefore be covered, especially since the flame is extinguished at far lower temperatures than in the case of a small backflow bubble. The reasons for this are the heat exchange between the backflow bubble and the ignitable fuel/air mixture and also the stabilization of the flame front in the flow zone.
The above comments show that a variation in output in the sense of an increase in output of a gas turbine plant merely by scaling up the overall size of a hitherto known premix burner leads to a multiplicity of problems and thus inevitably necessitates a completely new design of a conically designed premix burner known up to now. It is necessary to provide a remedy here and to search for measures in order to also permit desired scaling of gas turbine plants with the premix burners currently in operation and having a mixing section arranged downstream, and this with only slight constructional changes to existing premix burner systems.